Many semiconductor manufacturing processes use or generate flammable gases. For example, epitaxial deposition processes conducted within a processing chamber may utilize a silicon source gas, typically silane or one of the chlorosilane compounds, in a hydrogen atmosphere at high temperature, typically around 800-1100° C., and under a vacuum condition. Silane and ammonia may be supplied to a process chamber to form a thin film of silicon nitride on a substrate. As another example, a fuel gas may be added to a gas mixture used to etch a dielectric film.
A process tool typically has a plurality of process chambers, each of which may be at respective different stage in a deposition, etching or cleaning process, and so the gas being exhaust from the chambers at any given time may have various different compositions. In such processes, only a small proportion of the gases is consumed and so the majority of the gases supplied to the chambers is exhausted from the chambers together with solid and gaseous by-products from the processes.
The exhaust system for drawing the exhaust gases from the process chambers typically comprises a plurality of secondary pumps, each for drawing gas from a respective process chamber, and at least one primary pump backing the secondary pumps. Consequently, the exhaust gas streams drawn from the process chambers tend to be combined within a manifold or other connected piping within the exhaust system, bringing together the process gases and by-products from a number of different processes.
Whilst the combined exhaust gas itself tends not to be flammable, the pumping of such gases requires great care to be placed on the leak integrity of the exhaust system to ensure that there is no ingress of air. If the exhaust gas is above its lower explosive limit (LEL), any ignition sources within the exhaust system could result in the generation of hazardous flame fronts travelling through the exhaust system.
A common technique used to avoid ignition of a flammable gas stream is to introduce into the gas stream an excess of an inert purge gas, typically nitrogen. Vacuum pumps used in the exhaust systems connected to semiconductor processing chambers have historically been either oil filled pumps or multi-stage dry pumps. However, these pumps have a limited capability to take additional purge gas into their final stages to facilitate dilution of exhaust gas to a level below which an addition of an oxidising gas to the exhaust gas would be unable to raise the exhaust gas above its LEL.
Whilst a device may be provided in the exhaust system for controllably pyrolysing or otherwise removing any flammable gases contained within the exhaust gas, the presence of corrosive, acidic species in the exhaust gas can significantly reduce the lifetime and efficiency of the pyrolysing device. For example, acidic gases such as HF or HCl may be generated as by-products from a process conducted in the process chamber, and etching gases supplied to a process chamber may include halocompounds such as HCl, HBr, BCl3, Cl2 and Br2, and combinations thereof.